During radiation detection performed according to the photon counting method used at a high count rate in medical applications, in order to accurately obtain the energy of radiation photons, the photoelectric conversion gain of an avalanche photodiode (APD) for photon detection, which detects radiation photons, needs to be maintained at a constant level while performing the measurement.
An APD has a voltage higher than the breakdown voltage applied in between the anode and the cathode, and operates in a Geiger mode. The photoelectric conversion gain of an APD is proportional to the voltage applied excessively (the excessive voltage) against the breakdown voltage. On the other hand, the breakdown voltage varies according to the temperature. Hence, since the excessive voltage varies in response to a change in the surrounding temperature, the photoelectric conversion gain of the APD also varies. Thus, in order to perform measurement while keeping the photoelectric conversion gain of the APD at a constant level, it is necessary to keep a constant excessive voltage applied to the APD regardless of the temperature variation.
Meanwhile, a type of feedback control is known in which the surrounding temperature of an APD is detected using a temperature sensor, and the power-supply voltage of the APD is controlled according to the temperature variation using peripheral circuitry including a central processing unit (CPU).
However, in order to vary the inverse voltage which is applied to an APD according to the temperature, it is necessary to install a temperature sensor, a temperature sensor driving circuit, and a bias voltage varying circuit. Hence, the configuration becomes complex and the processing time also becomes long.